The present invention relates to the field of devices for joining electrical components to one another and, more particularly, to a method of forming a solder ball on a contact and to a method and apparatus for facilitating the soldering of a first electronic device, such as a connector, to a second electronic device, such as a printed circuit board.
It is often necessary and desirable to electrically connect one component to another component. For example, a multi-terminal component, such as a connector, is often electrically connected to a substrate, such as a printed circuit board, so that the contacts or terminals of the component are securely attached to contact pads formed on the substrate and/or to holes lined with an electroplating material in the substrate to provide an electrical connection therebetween. One preferred technique for securely attaching the component terminals to the contact pads and/or plated lining holes is to use a solder material.
When joining a multi-terminal component, such as a connector, to a substrate by soldering, particularly a substrate with internally plated holes, special provisions have often been required, such as shown in U.S. Pat. Nos. 4,597,625; 4,802,862; 4,883,435; 5,139,448; and 5,334,059, all of which are incorporated herein by reference in their entirety. Such components have terminals which do not carry solder, so that these situations have generally required special means for providing solder to the component terminals and to contact pads on the substrate. One approach to providing solder to the component terminals and contact pads is to provide solder paste in and around the particular area, such a hole. However, this approach generally does not provide a sufficient volume; of solder to properly join the component terminals and contact pads.
In the mounting of an integrated circuit (IC) on a substrate (e.g., formed of a plastic or a ceramic), the use of ball grid array (BGA) or other similar packages has become common. In a typical BGA, spherical solder balls attached to the IC package are positioned on electrical contact pads of a circuit substrate to which a layer of solder paste has been applied. The solder paste is applied using any number of techniques, including the use of a screen or mask. The unit is then heated to a temperature at which the solder paste and at least a portion or all of the solder balls melt and fuse to an underlying conductive pad formed on the circuit substrate. The IC is thereby connected to the substrate without need of external leads on the IC.
The BGA concept also offers significant advantages in speed, density, and reliability and as a result, the BGA package has become the packaging option of choice for high performance semiconductors. The inherent low profile and area array configuration provide the speed and density and the solid solder spheres provide enhanced solder joint reliability. Reliability is enhanced because the solder joints occur on a spheroid shape of solid solder. The spheroid shape, when properly filleted, provides more strength than flat or rectangular shaped leads of equivalent area. The solid solder composition provides a more reliable solder joint than conventional stamped and plated leads because there can be no nickel underplate or base metal migration to contaminate or oxidize the solderable surface, or weak intermetallic layers than can form when the solder bonds to a nickel underplate. Further, tin and tin plating processes used on conventional stamped and plated leads have additives than can inhibit solderability. Enhanced solder joint reliability is particularly important to an area array package because the solder joints cannot be visually inspected.
While the use of a BGA connector in connecting the IC to the substrate has many advantages, there are several disadvantages and limitations of such devices. It is important for most situations that the substrate-engaging surfaces of the solder balls are coplanar to form a substantially flat mounting interface so that in the final application, the solder balls will reflow and solder evenly to the planar printed circuit board substrate. If there are any significant differences in solder coplanarity on a given substrate, this can cause poor soldering performance when the connector is reflowed onto a printed circuit board. In order to achieve high soldering coplanarity, very tight coplanarity requirements are necessary. The coplanarity of the solder balls is influenced by the size of the solder balls and their positioning on the connector.
Conventional BGA connector designs attach loose solder balls to the assembled connector. The attachment process requires some type of ball placement equipment to place solder balls on a contact pad or recessed area of the connector that has been applied with a tacky flux or solder paste. The connector then goes through a reflow oven to solder the balls to the contact. The process is slow, sensitive, and requires expensive, specialized equipment.
An example of a BGA type connector is described in U.S. Pat. No. 6,079,991, (""991) to Lemke et al., which is herein incorporated by reference in its entirety. The connector includes a base section having a number of outer recesses formed on an outer surface of the base section. Similarly, the base section also has a number of inner recesses formed on an inner surface of the base section. The inner recesses are designed to receive contacts and the outer recesses are designed to receive solder balls so that the solder balls are fused to bottom sections of the contacts which extend into the outer recesses. The contacts comprise both ground/power contacts and signal contacts with top sections of the contacts providing an electrical connection with an electronic device by known techniques. Another electronic device, e.g., a PCB, is electrically connected to the contacts by soldering the solder balls onto contacts formed on the PCB, thereby providing an electrical connection between the two electronic devices.
While the ""991 connector is suitable for use in some applications, it suffers from several disadvantages. First, the connections between the solder balls and the bottom sections of the contacts may lack robustness and durability since the solder balls are simply placed in the outer recesses and then reflowed to form the electrical connection between the contact and one electronic device. Accordingly, only a portion of each solder ball is in contact with the bottom section of one contact before and after the soldering process. Second, because the solder balls are simply inserted into the outer recesses, the solder balls may not be coplanar with one another during the use of the connector and during the reflow process. Another disadvantage of this type of connector is that the solder joints are especially susceptible to fracturing during thermal expansion and cooling. The base section and the printed circuit board typically each has a different coefficient of thermal expansion and therefore when both are heated, one component will expand greater than the other. This may result in the solder joint fracturing because the solder ball is confined within the outer recess and the movement of the end of the contact to which the solder ball is attached is limited due to housing constraints. In other words, the contact is held in place within the housing substrate and only slightly protrudes into the recess where the solder ball is disposed. The contact therefore is effectively held rigid and not permitted to move during the reflow process.
In addition, the costs associated with manufacturing the ""991 connector are especially high since the contacts must be placed in the base section and then the individual solder balls must be placed within the outer recesses formed in the base section. A BGA type connector likely includes hundreds of solder balls and thus, the process of inserting individual solder balls into the outer recesses requires a considerable amount of time and is quite costly.
It is therefore desirable to provide a simple and inexpensive way of applying solder to contacts, e.g., terminal pins, which may be readily automated, without requiring separately applying solder paste to the component terminals or to the plated holes or contact pads of the substrate. It is also desirable to provide an alternative device and method for mounting high density electrical connectors on substrates, e.g., PCBs, by surface mounting techniques, e.g., using a ball grid array type connector.
The present invention provides a method of forming a solder ball on a portion, e.g., one end, of a contact. In one exemplary embodiment, the contact is a terminal pin which is intended for use in an electrical connector and more particularly, for use in a solder ball grid array (SBGA) connector. Generally and according to one embodiment, a solder-holding clip having a body with an opening is provided. The body has a solder-holding conformation adjacent the opening and a solder mass is positioned by the conformation. Preferably, the conformations of all of the embodiments act to hold the solder mass. The contact is then positioned proximate to the body opening and heat is applied to the solder mass causing the solder to reflow so that the solder flows into a spherical shape. This results in a solder ball being formed on the portion of the contact. Subsequent to forming the solder ball, the solder-holding clip is separated from the contact leaving a contact with a solder ball affixed thereto. Preferably, a series of solder-holding clips are provided along a carrier strip of material so that a number of solder balls may be formed on corresponding contacts during a single reflow operation.
According to the present invention, the solder-holding clip is formed of a non-wettable material. This permits the solder to reflow smoothly according to the natural flow tendencies of the solder material. The solder-holding clip is designed so that its geometry permits the solder material to flow naturally upon application of heat into a spherical shape, thereby forming the solder ball. The natural tendency of solder material which is subjected to heat is to reflow into a spherical body because of the various physical and thermal properties of the solder material. For example, the resulting surface tension of the solder material, as it undergoes this reflow operation, directs the solder material into the spherical body so long as its natural flow is unimpeded by any features of the clip. The present invention provides a variety of different types of solder-holding clips which are suitable for use because each clip has features which permit the solder to reflow into a spherical shape.
As previously-mentioned, contacts produced by the method of the present invention find particular utility in SBGA connector applications. An SBGA connector is used to electrically connect a first electronic device to a second electronic device. The connector includes a predetermined number of contacts which are disposed within a housing according to a predetermined arrangement. Each contact has a solder ball formed at one end of the contact and this type of solder ball contact provides the same advantages as other types of conventional solder ball grid array configurations.
The contacts may then be conveniently and easily disposed within openings formed in the housing and the coplanarity of the solder balls is controlled so that substrate-engaging surfaces of the solder balls are coplanar to form a substantially flat mounting interface. An opposite end of each contact is designed so that it may or may not separably connect to a terminal (contact) of the first electronic device and the solder ball formed at the end of the contact is disposed relative to a corresponding contact of the second electronic device. Preferably, the second electronic device is a printed circuit board and the contacts of the device are surface mount contact pads. Accordingly, each solder ball is disposed proximate to and preferably in intimate contact with one surface mount contact pad or solder paste on the pad prior to subjecting the connector to a final reflow operation. In the final reflow operation, each solder ball is heated so that the solder material flows onto and provides a secure electrical connection with the corresponding surface mount contact pad.
In another embodiment, the contacts are not loose but are already disposed within the connector prior to the formation of the solder balls on the ends of the contacts. In this embodiment, the connector is positioned relative to the solder-holding clip structure so that each contact is aligned proximate to one opening of a corresponding solder-holding clip. After a solder ball is formed at the end of each contact as a result of the reflow operation, the connector is removed from the solder-holding clip structure.
The connector of the present invention provides numerous advantages over conventional BGA connectors. For example, the connector of the present invention is a lower cost product that offers superior design and reliability compared to conventional devices. By eliminating the time intensive solder ball attachment process, the manufacturing cost and time are reduced. Quality and reliability are enhanced because the solder balls of the present connector are intimate and positive to the parent contact and lead coplanarity is improved and is more consistent. In another aspect of the present invention, the connector provides a compliant lead.
Thus, the present invention provides a robust solder ball contact by a simple yet effective method which takes advantage of the natural reflow characteristics of the solder material by using a solder-holding clip having a tailored geometry and by positioning the contact proximate to an opening formed in the solder-holding clip.
The above-discussed and other features of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.